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repetto74

TI99/4A Black screen

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Hi Guys,

 

Today I had to open my TI99/4A because there were a lot of video glitches on screen (top half part of the screen with a pink overlay and sometimes a lot of video glitches popping out). It turned out to be the cart connector making bad contacts. After a good cleaning and a test on the fly all seemed to work again but after re-assembling the machine then the TI99/4A will not power up anymore :-(. All I have is a black screen. I am maybe suspecting the power switch contacts having gone bad. How can I test it and how can I clean the switch? Is there any reference for a suitable replacement?

PS : The machine has the 32K mod hard wired in it and done by myself so could be also that a bad soldering somewhere is preventing a good start up.

 

Thanks

Rick

Edited by repetto74

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Little update. I found that when pushing hardly on the left side of the PCB board and firing up the machine then it will boot properly on the welcome screen! If I leave my finger then it goes black again. I tried then to wiggle gently the PCB and can see distorsions on screen. Sometimes when I put some pressure on the right side of the PCB then the video come back again. Weird and probably related to a broken track or bad solder joint? How can I proceed to find it out?

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Test are done without the cartridge port. I have some soldering skills and tools required so O would like to try to fix it :-). Thanks for the proposal but I am located in Europe and shipping costs will be far too high.

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The contacts on the DIN connector on the motherboard sometimes come lose with the plugging/unplugging of the modulator. Try resoldering the connections to that connector.

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Hi Dragonstomper,

 

This video speaks more than words on the issue I have. Putting pressure with my fingers on top of the clock generator and on the opposite edge of the board (close to the GROMS and Sound generator) will make the TI99 to boot up normally. As I release gently the pressure I start having the video getting garbled and when I totally release pressure than it goes black. Putting pressure again will make the TI99 boot sequence to start again (beep sound) so I do not think this is just related to a bad video connection. There is a cracked joint or track somewhere in the area where the finger pressure is bending the PCB. At first glance I cannot see anything from the solder side of the PCB.

 

<iframe width="560" height="315" src="https://www.youtube.com/embed/YMzSjsBfFKI"frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

 

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Is the clock chip socketed? Presumably that's a heatsink with white goo in the video? If socketed, pop it out, clean pins and replace. Does moving any of the wires (particularly the ones from the PSU) have the same effect as pressing on the PCB? The VDP is also socketed - probably worth removing and cleaning pins just to be sure.

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Hi Dragonstomper,

 

Yes the clock generator is socketed and was initially suspecting a bad socket so desoldered it properly and replaced with a precision socket before I made this video so no changes. Moving the PSU wires does not make any difference, I have checked voltages and they are all fine + applied fresh solder on the wires connection. Will try to clean the VDP socket with contact cleaner then.

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The bodger in me wants to suggest reassembling it in the case with a bit of folded cardboard positioned just where you need to apply pressure ... ;-)

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Test are done without the cartridge port. I have some soldering skills and tools required so O would like to try to fix it :-). Thanks for the proposal but I am located in Europe and shipping costs will be far too high.

YOU LIVE IN EUROPE?!?!?!

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The bodger in me wants to suggest reassembling it in the case with a bit of folded cardboard positioned just where you need to apply pressure ...  

I do not really catch what do you mean here? You mean putting cardboard to simulate the finger pressure? How can I do that ?

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Will try to clean the VDP socket with contact cleaner then.

Cleaning the VDP socket will not fix the problem. I have spotted the crack or bad solder joint somewhere around the two scratchpad rams I think.

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Solved finally!!!! :grin: I was applying some fresh solder on the suspect area and when trying to wiggle the PCB while powering on the TI99 I put my hand onto this large flat white resistor and everything started to move and beep on screen. So I put fresh solder on the resistor solder joint and all is back to business again :thumbsup: :grin:

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Solved finally!!!! :grin: I was applying some fresh solder on the suspect area and when trying to wiggle the PCB while powering on the TI99 I put my hand onto this large flat white resistor and everything started to move and beep on screen. So I put fresh solder on the resistor solder joint and all is back to business again :thumbsup: :grin:

 

That “large flat white resistor” sounds suspiciously like one of those above-the-board circuit connectors TI used in the TI-99/4A, which seemed to double as shields and/or airflow baffles.

 

..lee

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That “large flat white resistor” sounds suspiciously like one of those above-the-board circuit connectors TI used in the TI-99/4A, which seemed to double as shields and/or airflow baffles.

I misspelled the computer on my post above and it is a TI99/4A. There are two or three of those big resistors which looks more of a flat piece of cardboard. They can be bent quite easily when pushing on them with the risk of breaking the legs with repetitive torsions. Very bad choice I have to say. Solved anyway :-)

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Hi Guys,

 

I spoke too soon unfortunately. All the tiny legs of this long flat resistor are broken just at the edge and soldering the small remain into the through hole will not make contacts :-(. Is there any trick to fix them? Putting solder paste onto those tiny legs seems quiet difficult. Any idea if those resistor can be found to purchase?

post-37196-0-18360400-1544979552_thumb.jpg

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They're not resistors - they're just strips for distributing power to different parts of the PCB.

 

Use a multimeter to work out which pins are connected to which (there are probably at least two metal strips within the package, with some pins connected to each strip. Then double check it. Then triple check it. Then connect the equivalent pads on the PCB using some decent gauge wire. If you get the connections wrong, you'll blow some chips ...

 

Or alternatively solder a short length of wire through each hole in the PCB and solder the other end to the stump of the pin on the white strip. Just be careful that nothing can short out when you refit the PCB into the metal shell.

 

And stop bending them! ;-)

Edited by Stuart
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Yeh, sounds good, I have a TI99/4a motherboard, that was the one that my TIM occupied, it did the same thing, due to being stored in unstable locations, during the 18 year hiatus. I will be doing the same for it, hopefully sometime next year.

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